As part of the manufacturing process of semiconductor devices, semiconductor wafers are polished by CMP. The uniform removal of material from and the planarity of patterned and un-patterned wafers is critical to wafer process yield. Generally, the wafer to be polished is mounted on a wafer carrier which holds the wafer using a combination of vacuum suction or other means to contact the rear side of the wafer and a retaining lip or ring around the edge of the wafer to keep the wafer centered on the wafer carrier. The front side of the wafer, the side to be polished, is then contacted with an abrasive material such as an abrasive pad or abrasive strip. The abrasive pad or strip may have free abrasive fluid sprayed on it, may have abrasive particles affixed to it, or may have abrasive particles sprinkled on it.
The ideal wafer polishing process can be described by Preston's equation: R=K.sub.p *P*V, where R is the removal rate; Kp is a function of consumables (abrasive pad roughness and elasticity, surface chemistry and abrasion effects, and contact area); P is the applied pressure between the wafer and the abrasive pad; and V is the relative velocity between the wafer and the abrasive pad. As a result, the ideal CMP process should have constant cutting velocity over the entire wafer surface, constant pressure between the abrasive pad and wafer, and constant abrasive pad roughness, elasticity, area and abrasion effects. In addition, control over the temperature and pH is critical and the direction of the relative pad/wafer velocity should be randomly distributed over the entire wafer surface.
One common type of wafer polishing apparatus is the CMP model 372M made by Westech Systems Inc. A wafer is held by a wafer carrier of the model 372M. The wafer carrier rotates about the axis of the wafer. A large circular abrasive pad is rotated while contacting the rotating wafer and wafer carrier. The rotating wafer contacts the larger rotating abrasive pad in an area away from the center of the abrasive pad.
Another related apparatus is a polishing machine for polishing semiconductor wafers containing magnetic read-write heads, disclosed in U.S. Pat. No. 5,335,453 to Baldy et al. With this machine, a semiconductor wafer is held by a wafer carrier which is moved in a circular translatory motion by an eccentric arm. The wafer is polished by contacting an abrasive strip which is advanced in one direction. The relative motion between the wafer and the abrasive strip is a combination of the circular motion of the wafer and the linear motion of the advancing abrasive strip.
While the precessing circle polishing pattern should provide more uniform velocities such that different points on the wafer see similar velocities at any given time, the velocities are still not constant. Assuming the rotation of the eccentric arm is held to a constant angular speed, the precessing circle relative motion results in fluctuating velocities. When the wafer is rotating away from the precessing direction the net relative velocity is lower, and when the wafer is rotating with precessing direction the net relative velocity is higher.
Moreover, the apparatus has the disadvantage of not being able to provide alternative polishing patterns. Since the wafer carrier is mounted on a rotating eccentric arm, the wafer can only be polished by moving in a circle. Polishing patterns other than circular are desired for a number of reasons.
One such reason is to provide more uniform wear of the abrasive pad. Nonuniform wear of the abrasive pad results in a non-uniform removal rate of wafer material since more heavily worn sections of the abrasive pad remove material at a lower rate. Non-uniform wear also results in less efficient use of the abrasive pad itself, since the pad must be changed more often or advanced at a faster rate in order to avoid using portions of the pad which wear out first.
Many CMP wafer carriers currently available yield wafers having anomalies in planarity. Two pervasive problems that exist in most CMP wafer polishing apparatuses are underpolishing of the center of the wafer, and the inability to adjust the control of wafer edge exclusion as process variables change. For example, wafer carriers used on many available CMP machines experience a phenomenon known in the art as "nose diving". During polishing, the head reacts to the polishing forces in a manner that creates a sizable moment. This moment causes a pressure differential along the direction of motion of the head. The result of the pressure differential is the formation of a standing wave of the chemical slurry that interfaces the wafer and the abrasive surface. This causes the edge of the wafer which is at the leading edge of the wafer carrier, to become polished faster and to a greater degree than the center of the wafer.
The removal of material on the wafer is related to the chemical action of the slurry. As slurry is inducted between the wafer and the abrasive pad and reacts, the chemicals responsible for removal of the wafer material gradually become exhausted. Thus, the removal of wafer material further from the leading edge of the wafer carrier (i.e., the center of the wafer) experiences a diminished rate of chemical removal when compared with the chemical action at the leading edge of the wafer carrier (i.e., the edge of the wafer), due to the diminished activity of the chemicals in the slurry when it reaches the center of the wafer. This phenomenon is sometimes referred to as "slurry starvation".
Since the motion of the wafer is generally not linear but rotary, the wafers produced have generally been characterized by a domed or dished surface rather than the desired planar surface. Several attempts have been made to correct the domed or dished oxide removal patterns.
One such attempt was carried out by blowing air behind the wafer near its center. Theoretically, the air pressure would tend to slightly increase the pressure between the center of the wafer and the abrasives, thereby increasing the rate of abrasion at the center to match the rate of abrasion at the periphery of the wafer so as to form a planar product. However, the results of this process have proven unsatisfactory because of an inability to consistently control the pressure of the air trapped between the center of the wafer and the wafer carrier.
In another attempt, the wafer has been bonded around its periphery to a bladder on the wafer carrier to form a pocket in the center which can be filled with air to achieve the results attempted as described in the previous attempt. A problem with this approach is that the bladder is not sufficiently stiff to resist the polishing forces, leading to either failure of the seal which holds the air pocket in, or complete failure of the polishing process.
Still other attempts have been made to shape a film or carrier of a head with a slight crown or radius. This gives a desirable effect as long as none of the variables change during polishing. A major drawback is that the curvature of the crown or radius cannot be adjusted to adapt to changing variables in the process. Thus there is a need for a wafer carrier having a surface which can be adjustably controlled and maintained against a wafer to correct for anomalies in the abrasive removal of the wafer surface. Japanese Laid-Open Patent Application No. 8-39422 to Shendon discloses a wafer carrier for positioning a substrate with respect to a rotating polishing pad. A first chamber within the wafer carrier is provided with a bellows which is expandable for applying a first pressure onto the substrate. A second chamber is provided beneath the first chamber for applying a pressure to a lower contoured wall that interfaces with the substrate.
U.S. Pat. No. 5,205,082 to Shendon et al. discloses a CMP device which attempts to control the relationship between the platen, ring and pad by tying the platen and the ring together through a flexible diaphragm. However, by allowing the ring to float with respect to the platen, the ring can be upset by changes in abrasive pad flatness, roughness, and friction. When the ring is disturbed, the pressure on the periphery of the wafer increases. This can contribute to poor planarity because of more pronounced oxide removal rates near the wafer edges.